A wave is an oscillation, if represented in the plane using a Cartesian coordinate system looks like a sine wave, being the x axis time and the y axis amplitude. Amplitude is the variation of the wave across a given period being the peak the highest point of a wave. The difference between two peaks is a wavelength. Period is the time it takes a wavelength to pass by a point in space. Frequency is the number of waves that pass by in a second.

Black bodies - A cavity that absorbs all radiation and emits radiation at a specific frequency.

Resonant cavities - A cavity that allows standing waves of a specific frequency.

Sounds of frequency below 20Hz is called InfraSound can not be heard by human's ears

sound travels the earth so it can be used to detect nuclear blasts and allows whales to communicate for 10,000 miles (sound channel).

can be greater than >120 dB is intensity but not heard by people. associated with avalanches, earthquakes, volcanoes, meteors, the earth groaning, rock concerts, and pipe organs. Intensifies emotions.

Infrasound has some really warped effects on society.

sound that travels the earth

often >120 dB is intensity.

If heard would be a large rumble as loud as a jet engine or gunfire.

At rock concerts they play infrasound. This intensifies emotions. This is one possible reason why people play loud music.

Attraction of pipe organs.

In addition the earth groans. Anyone know the mythological significance of the cry of the earth?

used by whales to communicate as the waves can travel 10,000 miles.

Infrasound appears to heighten emotions and make people nervous and thus has an association with the supernatural such as haunted houses. Since people are emotionally attached to the deceased and go to where they died, when you combine it with infrasound which is associated with chills, is this an explanation for ghosts?

One of the stranger uses of infrasound involves combining an ancient chamber that resonates infrasound with mind altering drugs to induce visions and out of body experiences.

ELF is used by the US Navy to communicate with submerged submarines. The extremely high electrical conductivity of seawater shields submarines from most electromagnetic communications. Signals in the ELF frequency range, however, can penetrate much more deeply. The low transmission rate of most ELF communications limits their use as communications channels; generally an ELF signal serves to request that a submarine surface and initiate some other form of contact.

One of the difficulties posed when broadcasting in the ELF frequency range is antenna size. In order to transmit internationally using ELF frequencies, an extremely large antenna is required. The US maintains two sites, in Wisconsin and Michigan. Both sites use long power lines as antennae, in multiple strands ranging from 14 to 28 miles long. Considerable amounts of power are generated and emitted by ELF, and there have been some concerns over the possible ecological impact of such signals.

See also: TACAMO, HAARP, List of initialisms,

Starting at 7.8 Hz there is the Schumann resonance, which is due to the space between the surface of the Earth and the ionosphere. It acts as a resonant cavity fueled by energy from lightning strikes. Serial Experiments Lain had the idea that the Schumann resonance could be a carrier for a collective consciousness.

Very Low Frequency or VLF refers to Radio Frequencies (RF) in the range of 3 - 30 kHz. Since there is not much space in this band of the spectrum, only the very simplest signals are used, such as for radionavigation. Many natural radio emissions, such as whistlers, can be heard in this band.

Because VLF can penetrate water to a depth of 20 meters, they are used to communicate with submarines.

Lightning generates low frequency radio waves called whistlers. Whistlers have been recieved from Jupiter showing the presence of lightning there.

The dawn chorus is an unexplained electromagnetic phenomenon that occurs most often at sunrise or shortly after, that (with the proper radio equipment) resembles the sound of a massive flock of birds. It is thought to be caused by high-energy electrons that get caught in the Van Allen radiation belts of the Earth's magnetosphere and fall to the Earth's surface in the form of audible radio waves. Dawn choruses occur more frequently during magnetic storms.

This phenomenon also occurs during aurorae, when it is termed an auroral chorus.

Low Frequency or LF (sometimes called longwave) refers to Radio Frequencies (RF) in the range of 30-300 kHz. In Europe, part of the LF spectrum is used for AM broadcast service. In the western hemisphere, its main use is for aircraft beacon, navigation, information, and weather systems.

pass through ionosphere

can penetrate oceans to 200 meters

Long wave is 144 - 351 kHz in Europe, Africa, and Asia. In the Americas, this band is reserved for aeronautical navigation.

Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. They have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect nor refract using the ionosphere, so there are fewer phase-caused fadeouts.

In Europe, North Africa and Asia, longwave radio frequencies between 153 and 281 kHz are used for domestic and international broadcasting. In the Americas, frequencies between 200 and 430 kHz are used for non-directional radio beacons.

The frequency of 60 kHz is used by several nations, such as the United States, Germany, England, and Japan, for extremely accurate time and precision frequency signals. Many commercial appliances sold since approximately 2000 have a VLF receiver capable of receiving these signals, which penetrate indoors more effectively than mediumwave or shortwave signals.

Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 meters. The United States, Russian, British, Swedish, and Indian navies communicate with submarines on these frequencies.

Longwave transmitting antennas take up large amounts of space, and have been the cause of controversy in the United States and Europe due to fears over proximity to high-power radio waves.

(AM) Amplitude Modulation radio if from 530 - 1,700 kHz. Also known as medium wave. Was big from 1900's-1960's

AM radio technology is simpler than other types of radio, such as FM radio and DAB. An AM receiver detects the power of the radio wave, and amplifies changes in the power measurement to drive a speaker or earphones. The earliest crystal radio receivers used this principle.

AM radio was used for small scale voice and music broadcasts before World War I. The great increase in the use of AM radio came the following decade. The first commercial radio services began on AM in the 1920s. Radio programming boomed during the "Golden Age of Radio." Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.

Medium wave is by far the most used for commercial radio broadcasting; this is the "AM radio" that most people are familiar with.

For the long and medium wave bands, the wavelength is long enough that the wave diffracts around the curve of the Earth by ground wave propagation, giving AM radio, in particular long wave and medium wave at night, a long range.

Short wave is used by radio services intended to be heard great distances away from the transmitting station; the far range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different, see High frequency. AM is used mostly for broadcast uses - other shortwave users may use a modified version of AM such as SSB or an AM-compatible version of SSB such as SSB with carrier reinserted.

Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, amateur radio etc.

Since the ionosphere often reflects HF radio waves quite well, this range is extensively used for medium and long range terrestrial radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:

Shortwave radio operates between the frequencies of 3000 kHz and 30 MHz (30,000 kHz) and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. An alternate name is HF, or high frequency.

Short wavelengths are associated with high frequencies because there is an inverse relationship between frequency and wavelength.

Shortwave frequencies are capable of reaching the other side of the planet by bouncing a signal off the ionosphere. The selection of a frequency to use to reach a target area depends on several factors:

* The distance from the transmitter to the target receiver
* Time of day. During the day, higher shortwave frequencies (> 10 MHz) can travel longer distances than lower; at night, this : property is reversed.
* Season of the year.
* Solar conditions, including the number of sunspots, solar flares, and overall solar activity. Solar flares can prevent the : ionosphere from reflecting or refracting radio waves.

Some major users of the shortwave radio band include:

* Domestic broadcasting in countries with a widely-dispersed population with few longwave, mediumwave or FM stations serving them
* International broadcasting to foreign audiences
* Utility stations transmitting messages not intended for a general public, such as aircraft flying between continents, encoded or ciphered diplomatic messages, weather reporting, or ships at sea
* Amateur radio operators
* Time signal stations

The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave radio receivers in use in 2002.

The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunications Union, allocates bands for various services in conferences every few years. The next WRC is scheduled to take place in 2007. At the World Administrative Radio Conference (WARC) in 1997, the following bands were allocated to international broadcasters:

Shortwave broadcasting channels are allocated with a 5 kHz separation. International broadcasters, however, may operate outside the normal WARC-allocated bands or use off-channel frequencies to attract attention in crowded bands.

The power used by shortwave transmitters ranges from less than one watt for some experimental transmissions to 500 kilowatts and highter for intercontinental broadcasters. Shortwave transmitting centers often use specialized antenna designs to concentrate radio energy on a bearing aimed at the target area.

See International broadcasting for details on the history and practice of broadcasting to foreign audiences.

Amateur radio

The privilege of operating shortwave radio transmitters for non-commercial purposes is open to licensed amateurs. In the USA, they are licensed by the Federal Communications Commission (FCC). U.S. citizens do not need licenses to own or operate shortwave receivers. Recently the FCC has added an amateur radio license which requires no knowledge of Morse code, making it easier for beginners to get involved; however, a working knowledge of Morse code is required to operate on shortwave bands.

Amateur radio operators have made numerous technical advancements in the field of radio and make themselves available to transmit emergency communications when normal communications channels fail. Some amateurs practice operating off the power grid so as to be prepared for power loss.

Shortwave listening

Many hobbyists listen to shortwave broadcasters without operating transmitters. In many cases, the goal is to obtain as many stations from as many countries as possible (DXing); others listen to specialized shortwave utility, or "ute", transmissions such as maritime, naval, aviation, or military signals. Others focus on intelligence signals.

Shortwave listeners, or SWLs, can obtain "QSL" cards from broadcasters or utility stations as trophies of the hobby.

Numbers Stations

Numbers stations are shortwave radio stations of uncertain origin that broadcast streams of numbers, words, or phonetic sounds. Although officially there is no indication of their origin, radio hobbyists have determined that many of them are used by intelligence services as one-way communication to agents in other countries.

Shortwave radio: the future

The development of direct broadcasts from satellites has reduced the demand for shortwave receivers, but there are still a great number of shortwave broadcasters. A new digital radio technology, Digital Radio Mondiale, is expected to improve the quality of shortwave audio from very poor to standards comparable to the FM broadcast band. The future of shortwave radio is threatened by the uprise of power line communication (PLC), where a data stream is transmitted over unshielded power lines. As the frequencies used overlap with the shortwave bands severe distortions make listening to shortwave radio near power lines difficult or impossible.

Shortwave Broadcasts and Music

Some musicians have been attracted to the unique aural qualities of shortwave radio. John Cage employed shortwave radios as live instruments in a number of pieces, and other musicians have sampled broadcasts, used tape loops of broadcasts, or drawn inspiration from the unusual sounds on some frequencies. Karlheinz Stockhausen used shortwave radio in works including Telemusik (1966), Hymnen (1966-67) and Spiral (1968), and Holger Czukay, Pat Metheny, Aphex Twin, Meat Beat Manifesto, and Wilco have also used broadcasts.

Common uses for VHF are FM radio broadcast at 88-108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems (VOR in particular) and aircraft communications.

UHF and VHF are the most common frequency bands for television.

VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication. Unlike HF frequencies, the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and can't interfere with transmissions thousands of kilometres away) It is also less affected by atmospheric noise and interference from electrical equipment than low frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than higher frequencies. It was also easier to construct efficient transmitters, receivers, and antennas for it in the earlier days of radio. In most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as that operated by taxis and police), marine two-way audio communications, and aircraft radios.

The large slice of technically and commercially valuable slice of the VHF spectrum taken up by television transmission has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) in the next decade or so (currently scheduled for 2008 in the United States).

In the United Kingdom, the authorities chose to develop colour television exclusively on UHF, beginning in the late 1960s. The last British VHF TV transmitters closed down in 1986. VHF band III is now used in the UK for digital audio broadcasting.

New technology was added to FM radio in the early 1960s to allow FM stereo transmissions, where the frequency modulated radio signal is used to carry stereophonic sound, using the pilot-tone multiplex system.

This multiplexes the left and right audio signal channels in a manner that is compatible with mono sound, using a sum-and-difference technique to produce a single "mono-compatible" signal, which has a baseband part that is equal to the sum of the left and right channels (L+R), and a higher-frequency part that is the difference of the left and right channels (L-R) amplitude modulated on a 38 kHz subcarrier. A 19 kHz pilot tone is then added, to allow allow receivers to detect the presence of a stereo-encoded signal.

This signal can then be passed through the FM modulation and demodulation process as if it was a monophonic signal, and the stereo signals extracted from the demodulated FM signal by reversing the multiplexing process.

Simple mono FM receivers will not extract the left and right signals, but simply reproduce the baseband part of the "mono-compatible" signal. (This relies on the fact that the subcarrier-modulated part of the mono-compatible signal is in a part of the audio spectrum that is inaudible to people, and the pilot tone is a low-intensity tone in a part of the audio spectrum that is inaudible to most people).

This backwards compatibility was important, as when the FM stereo system was introduced in the U.S. in the 1960s, mono FM transmissions had been in service since the 1940s, and there was a large installed base of mono receivers that needed to be able to receive stereo broadcasts without any modification.

Stereo receivers could automatically switch between "mono" and "stereo" modes based on the presence of the pilot tone. They were also equipped with a notch filter to remove the pilot tone. In poor signal conditions, stereo receivers could also fall back to mono mode, even on a stereo signal, allowing improved signal-to-noise performance in these conditions.

The stereo multiplexing system has been further extended to add an extra, even higher frequency, 57 kHz subcarrier, which is used to carry low-bandwidth digital Radio Data System information, allowing digitally controlled radios to provide extra features.

FM radio channel assignments in the US

In the United States, frequency-modulated broadcasting stations operate in a frequency band extending from 87.8 MHz to 108.0 MHz, for a total of 20.2 MHz. It is divided into 100 channels, each 0.2 MHz wide, designated "channel 200" through "channel 300."

To receive a station, an FM receiver is tuned to the center frequency of the station's channel. The lowest channel, channel 200, extends from 87.8 MHz to 88.0 MHz; thus its center frequency is 87.9 MHz. Channel 201 has a center frequency of 88.1 MHz, and so on, up to channel 300, which extends from 107.8 to 108.0 MHz and has a center frequency of 107.9 MHz.

Because each channel is 0.2 MHz wide, the center frequencies of adjacent channels differ by 0.2 MHz. Because the lowest channel is centered on 87.9 MHz the tenths digit of the center frequency of any FM station in the United States is always an odd number. FM audio for television channel 6 is broadcast at a carrier frequency of 87.75 MHz, and many radios can tune down this low; a few low-power television stations licensed for channel 6 are operated solely for their right to use this frequency and broadcast only nominal video programming. For the same reason, assignment restrictions between TV stations on channel 6 and nearby FM stations are stringent: there are only two stations in the United States (KSFH and translator K200AA) licensed to operate on 87.9 MHz.

FM stations in a market are generally spaced four channels (800 kHz) apart. This spacing was developed in response to problems perceived on the original FM band, mostly due to deficiencies in receiver technology of the time. With modern equipment, this is widely understood to be unnecessary, and in many countries shorter spacings are used. Other spacing restrictions relate to mixing products with nearby television, air-traffic control, and two-way radio systems as well as other FM broadcast stations. The most significant such taboo restricts the allocation of stations 10.6 and 10.8 MHz apart, to protect against mixing products which will interfere with an FM receiver's standard 10.7 MHz intermediate frequency stage.

Commercial broadcasting is licensed only on channels 221 through 300, with 200 through 220 being reserved for noncommercial educational broadcasting. In some markets close to the Canadian or Mexican border, such as Detroit, Michigan and San Diego, California, commercial stations operating from those countries target U.S. audiences on "reserved band" channels, as neither Canada nor Mexico has such a reservation.

UHF frequencies have higher attenuation from atmospheric moisture and benefit less from 'bounce', or the reflection of signals off the ionosphere back to earth, when compared to VHF frequencies. The frequencies of 300-3000 MHz are always at least an order of magnitude above the MUF (Maximum Usable Frequency). The MUF for most of the earth is generally between 25-35 MHz. Higher frequencies also benefit less from ground mode transmission. However, the short wavelengths of UHF frequencies allow compact receiving antennas with narrow elements; many people consider them less ugly than VHF-receiving models

In the United States, UHF stations (broadcast channels above 13) originally gained a reputation for being more locally owned, less polished, less professional, less popular, and for having a weaker signal than their VHF counterparts (channels 2-13). The movie UHF, starring Weird Al Yankovic, parodies this phenomenon. Recently, with the emergence of eight major broadcast television networks, that notion has changed as bigger and bigger media companies seek a bigger slice of the television pie. Many Fox, UPN, WB, and Pax network affiliates broadcast in the UHF band.

As cable television, digital television, and DSS have penetrated the television market, the distinction between VHF and UHF stations has dissipated.

In Australia, UHF was first anticipated in the mid 1970's with channels 28 to 69. The first UHF TV broadcasts in Australia were operated by Special Broadcasting Service (SBS) on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for the Australian Broadcasting Corporation (ABC). The UHF band is now used extensively as ABC, SBS, commercial and community (public access) television services have expanded particularly through regional areas.

Note: above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Uses

A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.4 GHz for the purpose of cooking food. Microwaves cook food by causing molecules of water and other compounds to vibrate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.

A maser is a device similar to a laser, except that it works at microwave frequencies. Microwaves are also used in satellite transmissions because this frequency passes easily through the Earth's atmosphere with less interference than higher wavelengths.

Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.

Wireless LAN communication protocols such as IEEE 802.11 and bluetooth also use microwaves in the 2.4 GHz ISM band, although some variants use a 5 GHz band for communication.

Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies.

Microwaves can be used to transmit power over long distances, create a solar array to beam power to earth.

The microwave spectrum is defined as electromagnetic energy ranging from approximately 300 MHz to 1000 GHz in frequency. Most common applications are within the 1 to 40 GHz range.

Microwave Frequency Bands are defined in the table below:

L Band 1 to 2 GHz S Band 2 to 4 GHz C Band 4 to 8 GHz X Band 8 to 12 GHz Ku Band 12 to 18 GHz K Band 18 to 26 GHz Ka Band 26 to 40 GHz Q Band 30 to 50 GHz U Band 40 to 60 GHz V Band 46 to 56 GHz W Band 56 to 100 GHz

For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the following figures:

The Cosmic Microwave Background Radiation is a form of electromagnetic radiation that fills the whole of the universe. It has the characteristics of black body radiation at a temperature of 2.726 kelvins. It has a frequency in the microwave range.

CMR and the Big Bang

This radiation is regarded as the best available evidence of the Big Bang (BB) theory and its discovery in the mid-1960s marked the end of general interest for alternatives such as the steady state theory. The CMR gives a snapshot of the Universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 300,000 years after the Big Bang -- this point in time is generally known as the "last scattering surface" -- the temperature of the Universe was about 6000 K. Since then it has dropped because of the expansion of the Universe, which cools radiation inversely proportional to the fourth power of the Universe's scale length.

Features

One of the microwave background's most salient features is a high degree of isotropy. There are some anisotropies, the most pronounced of which is the dipole anisotropy at a level of about 10-4 at a scale of 180 degrees of arc. It is due to the motion of the observer against the CBR, which is some 700 km/s for the Earth.

Variations due to external physics also exist; the Sunyaev-Zel'dovich Effect is one of the major factors here, in which an cloud of high energy electrons scatters the radiation transferring some energy to the CMB photons.

Even more interesting are anisotropies at a level of roughly 1/100000 and on a scale of a few arc minutes. Those very small variations correspond to the density fluctuations at the last scattering surface and give valuable information about the seeds for the large scale structures we observe now. These density fluctations arise because different parts of the universe are not in contact with each other.

In addition, the Sachs-Wolfe effect causes photons from the Cosmic microwave background to be gravitationally redshifted. These small-scale variations give observational constraints on the properties of universe, and are therefore one important test for cosmological models.

Detection, Prediction and Discovery

The CBR was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in 1964 by Penzias and Wilson, who received a Nobel Prize for this discovery. The CBR had, however, been detected and its temperature deduced in 1941, seven years before Gamow's prediction. Based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote: "It can be calculated that the 'rotational' temperature of interstellar space is 2 K."

Because water absorbs microwave radiation, a fact that is used to build microwave ovens, it is rather difficult to observe the CMB with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments.

Experiments

Of these experiments, the Cosmic Background Explorer (COBE) satellite that was flown in 1989-1996 is probably the most famous and which made the first detection of the large scale anisotropies (other than the dipole). In June 2001, NASA launched a second CBR space mission, WMAP, to make detailed measurements of the anisotropies over the full sky. Results from this mission provide a detailed measurement of the angular power spectrum down to degree scales, giving detailed constraints on various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) [ed. see links below],

A third space mission, Planck, is to be launched in 2007. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency).

CBR and Non-Standard Cosmologies

During the mid-1990's, the lack of detection of anisotropies in the CBR led to some interest in nonstandard cosmologies (such as plasma cosmology) mostly as a backup in case detectors failed to find anisotropy in the CBR. The discovery of these anisotropies combined with a large amount of new data coming in has greatly reduced interest in these alternative theories.

Some supporters of non-standard cosmology argue that the primodorial background radiation is uniform (which is inconsistent with the big bang) and that the variations in the CBR are due to the Sunyaev-Zel'dovich effect mentioned above (among other effects).

Note: above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Infra-red means below red, where red is the color with the longest wavelength.

(FIR) far-IR (30-1000 µm)

(MIR/IIR) mid-IR (5-30 µm) - Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body).

(NIR) near-IR (0.7-5 µm)

However, these terms are not precise, and are used differently in various studies.

Uses Infrared is used in night-vision equipment, when there is insufficient visible light to see an object. The radiation is detected and turned into an image on a screen, hotter objects showing up brighter, enabling the police and military to chase targets. "Night vision"

Smoke is more transparent to infrared than to visible light, so fire fighters use infrared imaging equipment when working in smoke-filled areas. Fire fighters also use this equipment in wood-frame buildings after a fire has been extinguished to look for hot spots behind the walls, where a fire can break out again.

A more common use of IR is in television remote controls. In this case it is used in preference to radio waves because it does not interfere with the television signal. IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from sunlight, people and other warm objects.

The light used in fiber optic communication is typically infrared.

heat based

not really. EM waves are constantly being emitted by objects, black-body radiation, but the frequency depends on temperature. for room temperature objects and humans, it is mostly below light in the infrared, but for things like fires and red hot iron, it is in the light spectrum more, and for colder and hotter objects it is at different frequencies.

how about this, anything that has at least as much energy as infared will warm things up. Anything that has less will not warm atoms up.

Visible light exists on the tip end of infared and just below it is UV. Basically you can almost get a sunburn from visible light.

The optical spectrum (visible light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. The optical spectrum is a composite, or mixture, of the various colors.

There are no exact bounds to the optical spectrum ; a light-adapted eye typically has a maximum sensitivity of ~555 nm (in the green). Commonly the response of the eye is considered to cover 380 nm to 780 nm although a range of 400 nm to 700 nm range is more common. The eye may, however, have some visual response at even wider wavelength ranges.

Wavelengths in the range visible to the eye occupy most of the "optical window", a range of wavelengths that are easily transmitted through the Earth's atmosphere.

Note: Ultraviolet and Infrared are often considered to be "light" but are generally not visible to the human eye.

Visible Light

Visible light is electromagnetic radiation that is made of colors of light that the eye can see. This light has wavelengths that are generally expressed in nanometers.

See Rydberg formula.

Vision Rods and cones

People see red, yellow, green, blue and purple

People don't notice the difference because red and purple both have a low intensity.

Ultra-Violet means beyond violet, where violet is the color with the shortest wavelength.

It is colloquially called black light, as it is invisible to the human eye.

The sun emits ultraviolet light in the UV-A, UV-B, and UV-C bands

380-200 nm

near UV

380-315 nm

UV-A

99% of the UV light that reaches the Earth's surface.

315-280 nm

UV-B

Associated with skin cancer.

280-10 nm

UV-C

Absorbed by ozone layer, creates ozone layer.

200-10 nm

vacuum/extreme UV

Ordinary glass is transparent to UV-A but is opaque to shorter wavelengths. This is why people don't get sunburns in cars. During the first nuclear blast, Feynman was the only person who saw it because he looked through a window.

Quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.

In general, UV-A is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause sun burn. Because it does not cause reddenning of the skin (erythema) it can not be measured in the SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that your sunscreen block both UVA and UVB.

High intensities of UV-B light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye).

UVA, UV-B and UV-C all can damage collagen fibers and thereby accelerate aging of the skin.

Tungsten-Halogen lamps have bulbs made of quartz, not of ordinary glass. Tungsten-Halogen lamps that are not filtered by an additional layer of ordinary glass are a common, useful, and possibly dangerous, source of UV-B light.

UV-A light is known as "dark-light" and, because of its longer wavelength, can penetrate most windows. It also penetrates deeper into the skin than UV-B light and is thought to be a prime cause of wrinkles.

UV-B light in particular has been linked to skin cancers such as melanoma. The radiation ionizes DNA molecules in skin cells, causing covalent bonds to form between adjacent thymine bases, producing thymidine dimers. Thymidine dimers do not base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to mutations, which can result in cancerous growths. The mutagenicity of UV radiation can be easily observed in bacteria cultures.

This cancer connection is the reason for concern about ozone depletion and the ozone hole.

UV-C rays are the strongest, most dangerous type of ultraviolet light. Little attention has been given to UV-C rays in the past since they are normally filtered out by the ozone layer and do not reach the Earth. Thinning of the ozone layer and holes in the ozone layer are causing increased concern about the potential for UVC light exposure, however.

As a defense against UV light, the body tans when exposed to moderate (depending on skin type) levels of radiation by releasing the brown pigment melanin. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that partly blocks UV is widely available (often referred to as "sun block" or "sunscreen"). Most of these products contain an "SPF rating" that describes the amount of protection given. This protection applies only to UV-B light. In any case, most dermatologists recommend against prolonged sunbathing.

A positive effect of UV light is that it induces the production of vitamin D in the skin. Grant (2002) claims tens of thousands of premature deaths occur in the U.S. annually from cancer due to insufficient UV-B exposures (apparently via vitamin D deficiency).

Astronomy

In astronomy, very hot objects preferentially emit UV light (see Wien's law). However, the same ozone layer that protects us causes difficulties for astronomers observing from the earth, so most UV observations are made from space. (See UV astronomy, space observatory).

Uses UV light has many various uses:

Fluorescent lamps Fluorescent lamps produce UV light by the emission of low-pressure mercury gas. A phosphorescent coating on the inside of the tubes absorbs the UV and turns it into visible light.

The main mercury emission wavelength is in the UV-C range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous.

Disinfecting drinking water Ultraviolet light is increasingly being used to disinfect drinking water and in waste water treatment plants. Dr. James R. Bolton discovered that ultraviolet light could treat Cryptosporidium, previously unknown. The findings resulted in two US patents and the use of UV light as a viable method to treat drinking water.

Analyzing minerals Ultraviolet lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light; or may fluoresce differently under short wave ultraviolet versus long wave ultra violet. UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The fluorescent protein Green Fluorescent Protein (GFP) is often used in genetics as a marker. Many substances, proteins for instance, have significant light absorption bands in the ultraviolet that are of use and interest in biochemistry and related fields. UV-capable spectrophotometers are common in such laboratories.

Sterilization Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, however, these lamps are used only as a supplement to other sterilization techniques.

Resolution Ultraviolet light is used for very fine resolution photolithography, as required for manufacture of semiconductors.

Spectroscopy UV light is often used in UV-visible spectroscopy

Photolithography UV light is used extensively in the electronics industry in a procedure known as photolithography, where a chemical known as a photoresist is exposed to UV light which has passed through a mask. The light allows chemical reactions to take place in the photoresist, and after development (a step that either removes the exposed or unexposed photoresist), a geometric pattern which is determined by the mask remains on the sample. Further steps may then be taken to "etch" away parts of the sample with no photoresist remaining.

It is photolithography which is primarily used to create integrated circuit components and printed circuit boards.

Other The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150-200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as Circular Dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region.

It is advisable to use protective eyewear when working with ultraviolet light, especially short wave ultraviolet. Ordinary eyeglasses give some protection. Most plastic lenses give more protection than glass lenses. Some plastic lens materials, such as polycarbonate, block most UV. There are protective treatments available for eyeglass lenses that need it to give better protection. The most important reason that ordinary eyeglasses only give limited protection, however, is that light can reach the eye without going through the lens. Full coverage is important if the risk from exposure is high. Full coverage eye protection is usually recommended for high altitude mountaineering, for instance. Mountaineers are exposed to higher than ordinary levels of UV light, both because there is less atmospheric filtering and because of reflection from snow and ice.

Some insects, such as bees, can see into the near ultraviolet, and flowers often have markings visible to such pollinators.

X-rays are primarily used for diagnostic medical imaging and crystallography.

Physicist Johann Hittorf observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugene Goldstein. Later, English physicist William Crookes investigated the effects of energy discharges on rare gases. He constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminum). Philip Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration of X-rays through various materials. Philip Lenard, though, did not realize that he was producing X-rays.

In April 1887, Nikola Tesla began to investigate X-rays using his own devices as well as Crookes tubes. Tesla did this by experimenting with high voltages and vacuum tubes. From Nikola Tesla's technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube. Tesla's tubes differ from other X-ray tubes in that they have no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences. The modern term for this process is the bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments; however, he did not categorize the emissions as what was later called X-rays (generalizing the phenonomena as radiant energy). Tesla did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.

Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Roentgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII); however, he did not work with actual X-rays.

On November 8, 1895, Wilhelm Röntgen, a German scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as Röntgen rays in some countries. Roentgen received the first Nobel Prize in Physics for his discovery.

In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays. He found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped x-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life.[1]

In 1906, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel prize for this discovery.

The use of X-rays for medical purposes was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis.

Detectors

The detection of X-rays is based on various methods. Most commonly known is the photographic plate as we know them from hospitals. Initially, most common detection was based on the ionisation of gasses. One of the most simple examples is the Geiger counter. Since the 1990-ies, new detectors were developed based on semiconductors. In the semiconductor, the X-ray photon was converted to electron-hole pairs which were collected to detect the X-ray

Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation (see Electromagnetic spectrum) produced by radioactivity or other nuclear or subatomic processes such as electron-positron annihilation. Gamma rays are more penetrating than either alpha or beta radiation, but less ionizing. Gamma rays are distinguished from X rays by their origin. Gamma rays are produced by nuclear transitions while X-rays are produced by energy transitions due to accelerating electrons. Because it is possible for some electron transitions to be of higher energy than nuclear transition, there is an overlap between low energy gamma rays and high energy X-rays.

Gamma rays from nuclear fallout would probably cause the largest number of casualties in the event of the use of nuclear weapons in a nuclear war. An effective fallout shelter reduces human exposure at least 1000 times.

Gamma rays are less ionising than either alpha or beta rays. However, reducing human danger requires thicker shielding. They produce damage similar to that caused by X-rays such as burns, cancer, and genetic mutations.

In terms of ionization, gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering, and pair production.

Photoelectric Effect: This describes the case in which a gamma photon interacts with and transfers all of its energy to an orbital electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron. The photoelectric effect is thought to be the dominant energy transfer mechanism for x-ray and gamma ray photons with energies below 50 keV (thousand electronvolts), but it is much less important at higher energies.

Compton Scattering: This is an interaction in which an incident gamma photon loses enough energy to an orbital electron to cause its ejection, with the remainder of the original photon's energy being emitted as a new, lower energy gamma photon with an emission direction different from that of the incident gamma photon. The probability of Compton scatter decreases with increasing photon energy. Compton scattering is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV (million electronvolts), an energy spectrum which includes most gamma radiation present in a nuclear explosion. Compton scattering is relatively independent of the atomic number of the absorbing material.

Pair Production: By interaction in the vicinity of the coulomb force of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is a positively charged electron. Energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. The electron of the pair, frequently referred to as the secondary electron, is densely ionizing. The positron has a very short lifetime. It combines within 10−8 second with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each.

Gamma rays are often produced alongside other forms of radiation such as alpha or beta. When a nucleus emits an α or β particle, the daughter nucleus is sometimes left in an excited state. It can then jump down to a lower level by emitting a gamma ray in much the same way that an atomic electron can jump to a lower level by emitting ultraviolet radiation.

Gamma rays, x-rays, visible light, and UV rays are all forms of electromagnetic radiation. The only difference is the frequency and hence the energy of the photons. Gamma rays are the most energetic. An example of gamma ray production follows.

First cobalt-60 decays to excited nickel-60 by beta decay:

Then the nickel-60 drops down to the ground state (see nuclear shell model) by emitting a gamma ray:

Uses:

The powerful nature of gamma-rays have made them useful in the sterilising of medical equipment by killing bacteria. They are also used to kill bacteria in foodstuffs to keep them fresher for longer.

In spite of their cancer-causing properties, gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to focus the radiation on the growth while minimising damage to the surrounding tissues.